Think about your first experience volunteering. I would wager a bet to say that either you have a personal connection with the institution and wanted to help, or someone you know asked you to get involved.
In October, 2008, as part of an internship, I traveled to Kinshasa in the Democratic Republic of the Congo. An orphan I spoke to said the most important people in his life were those who helped. When I asked him who helped, he said no one.
You don't necessarily have to be a student to intern, and there are other ways of sampling careers. Do volunteer work. Help a friend, whose work intrigues you, on the weekends. I know people who tried on a new career just by entering a contest.